In the sorting of particulate material e.g., radioactive ore, it is necessary to make a grade assessment or measurement of each particle to arrive at a decision on whether to accept or reject the particle. The grade of a particle is essentially a measure of its radioactivity per unit mass and normally is determined by making a volume measurement of the particle, relating the volume directly to its mass, and calculating the ratio of a radioactive count produced by the particle to its mass.
This process is generally acceptable, without adjustment, when the ore is highly radioactive, but various errors due inter alia to the relative sizes of the particles, their densities and their shapes, become significant as the grade decreases and can result in erroneous accept or reject decisions.
To calculate the grade of radioactive material in the particle it is assumed that the count accumulated by radiation detectors during the passage of a specific particle past the detectors is directly proportional to the content of radioactive material in the particle, within the statistical limits of the random nature of emission of radiation by the radioactive material in the particle. It is however only true for a constant size, shape and mass of particle which factors affect the counting geometry as seen by the radiation detectors, and also the self absorption of radiation within the particle. The counting geometry and self absorption of radiation within the particle are extremely dependent on the shape and mass of the particle, so that for a constant mass of radioactive material in a particle, the counts accumulated by the detectors for that specific particle will vary very considerably with the mass of the particle and will not be constant as is assumed for the calculated grade. In practice, it is found that these factors can produce an error of 100% in the calculated grade of a particle with a mass of 50 gm as compared to a particle with a mass of 250 gm.